Silver-containing direct thermographic imaging materials are non-photosensitive materials that are used in a recording process wherein images are generated by the direct application of thermal energy. These materials have been known in the art for many years and generally comprise a support having disposed thereon one or more imaging layers comprising (a) a relatively or completely non-photosensitive source of reducible silver ions, (b) a reducing agent composition (acting as a developer) for the reducible silver ions, and (c) a suitable hydrophilic or hydrophobic binder. Thermographic materials are sometimes called “direct thermal” materials in the art because they are directly imaged by a source of thermal energy without any transfer of the energy or image to another material.
In a typical thermographic construction, the image-forming layers are based on silver salts of long chain fatty acids. The preferred non-photo-sensitive reducible silver source is a silver salt of a long chain aliphatic carboxylic acid having from 10 to 30 carbon atoms, such as behenic acid or mixtures of acids of similar molecular weight. At elevated temperatures, the silver of the silver carboxylate is reduced by a reducing agent whereby a black-and-white image of elemental silver is formed.
Problem to be Solved
Direct thermographic materials are imaged by a recording process wherein images are generated by imagewise heating a recording material containing chemical components that change color or optical density in an imagewise fashion. The chemical components include the reducing agent noted above. Many compounds have been described in the art that are considered useful for this purpose. They are often phenolic compounds having at least one hydroxy substituent as described, for example, in U.S. Pat. No. 5,582,953 (Uyttendaele et al.) and U.S. Pat. No. 6,093,528 (Terrell et al.).
The phenolic reducing agents of the prior art have varying usefulness, and vary in their light stability, resistance to aerial oxidation, tint and/or tone of the silver image, and ability to produce a dense black metallic silver image under the short time and high temperature conditions that occur during thermal printing. Thus, there remains a continuing need to provide reducing agents for direct thermographic materials that generate a dense black metallic silver image with a thermal print-head.